FI77642B - FOERFARANDE FOER ATT SEPARERA KVAEVE FRAON NATURGAS. - Google Patents

Description

77642

This invention relates to a process for the separation of nitrogen from natural gas and can be applied in particular to the separation of a stream from a source into methane, liquids from natural gas and nitrogen. Förfarande for att separera kväve frän Naturgas

As hydrocarbon resources dwindle and recovery becomes more difficult, the use of secondary recovery processes is spreading. Such secondary recovery processes are commonly referred to as Enhanced Oil Recovery Process (EOR) and Enhanced Gas Recovery Process (EGR). One such secondary recovery technology involves injecting a gas that does not sustain combustion into the source to increase the pressure in the source so that those hydrocarbons that cannot be removed from the source at its natural pressure can be removed. The gas commonly used in this process is nitrogen because it is relatively abundant and inexpensive, and because it can be produced in large quantities in the vicinity of the source.

After a certain period of time, the nitrogen injected into the source begins to escape with the hydrocarbons. Therefore, nitrogen must be removed from the hydrocarbon stream in order to meet the requirements for the minimum permissible heat content or the maximum permissible inert gas content of the fuel gas in the product.

In conventional methods for removing nitrogen from natural gases, often referred to as nitrogen removal processes, the stream from the source is separated into methane, nitrogen, and hydrocarbons having two or more carbon atoms, such as hydrocarbons are often referred to as natural gas liquids. This separation is carried out in conventional methods by first separating the stream from the source into a liquid stream, which 2 77642 comprises mainly natural gas liquids, and into a steam stream consisting mainly of nitrogen and methane. The liquid stream is recovered and the steam stream is cryogenically separated in one or more concentration columns into nitrogen and methane. When a single concentration column is used to perform cryogenic concentration, this column often operates with a heat pump provided by a recyclable liquid. The liquid in this heat pump is usually methane, as it can satisfy the temperatures required in the column boiler and condenser at reasonable pressure values.

One difficulty with such conventional methods is that some of the natural gas liquids do not separate in the first separation stage, but instead remain in methane and are recovered with the methane. This is disadvantageous because natural gas liquids can be used more economically other than as fuel, which in turn is methane. Thus, it would be desirable to recover more natural gas from liquids separated from methane than is possible with conventional nitrogen abandonment processes and use.

Another difficulty with these conventional methods is that the use of methane as a heat pump liquid in a cryogenic nitrogen and methane separation column requires either that the column be operated at relatively high pressure or that the heat pump circuit otherwise operate under vacuum conditions on its low pressure side. Vacuum conditions in a methane circuit are undesirable, firstly because the vacuum is in itself an uncertain mode of operation due to the possible absorption of air into the circuit, and secondly because the vacuum conditions require a relatively high power supply because considerable pumping energy is required. However, the operation of a cryogenic distillation column of nitrogen and methane at high pressure to avoid vacuum conditions in the heat pump circuit is disadvantageous because the higher pressure causes degradation in the column in the form of increased separation zones or recovery of the cooled liquid.

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3 77642 ta. Thus, it would be desirable for the nitrogen and methane distillation column to be able to operate at lower pressure without the heat pump circuit having to operate under vacuum conditions.

Accordingly, it is an object of the present invention to provide an improved process for removing nitrogen from natural gases.

Another object of the present invention is to provide an improved process for removing nitrogen from natural gases by which a greater amount of natural gas liquids are recovered separately from methane than is possible using conventional nitrogen removal processes.

It is a further object of the present invention to provide an improved process for removing nitrogen from natural gas in which the cryogenic nitrogen and methane separation column operates at lower pressures than usual, while avoiding the requirement for the heat pump to operate under vacuum conditions.

Summary of the Invention

The above and other objects which will become apparent to a person skilled in the art upon reading this publication are achieved by the method of the present invention, one feature of which is:

A process for removing nitrogen from natural gas, in which the nitrogen-containing hydrocarbon stream is first separated into a liquid containing mainly hydrocarbons having two or more carbon atoms and a vapor containing mainly nitrogen and methane, and in which the steam stream is further separated by one or more concentrations. to methane, the process improvement comprising: A) cooling the steam after the first separation to at least 10 degrees Kelvin to partially condense the steam; 4 77642 B) passing the condensed part to an evaporation column, where it is separated into a liquid containing mainly hydrocarbons having two or more carbon atoms and a vapor containing mainly methane; C) recovering the liquid hydrocarbon and methane vapor from step B); and D) passing the non-shrinkable portion to the concentration column (s), where it is separated into nitrogen and methane.

Another feature of the process of this invention is:

A cryogenic concentrating process for separating nitrogen from natural gas, comprising: a) passing a nitrogen-containing natural gas stream to a concentrating column operating at an absolute pressure in the range of 200 to 450 psi; (b) separating this stream in said column into nitrogen-enriched peak vapor and methane-enriched bottom liquid; c) cooling the peak steam by indirect heat exchange, wherein the heat pump flow medium comprising 0.5 to 60 mole percent nitrogen and 99.5 to 40 mole percent hydrogen is evaporated to partially condense said peak steam; d) returning the condensed portion of the peak vapor to the column as a reflux liquid and the process of removing the uncondensed portion of the vapor from the peak as nitrogen; e) heating the bottom liquid by indirect heat exchange, wherein said heat pump flow medium is condensed to partially vaporize said bottom liquid; and

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5 77642 (f) returning the evaporated part of the bottom liquid to the column as refrigerated steam and recovering the non-evaporated part as a natural gas product? in which process said heat pump flow medium is constantly at or above ambient pressure.

As used herein, the term "column" means a distillation or fractional distillation column, i.e., a contact-based column or zone in which liquid and vapor are brought into contact as they move countercurrently to cause a mixture of fluids to separate, e.g. on spaced bottoms or plates mounted inside the column, or alternatively on the surface of the filler pieces with which the column is filled. For a more detailed review of fractionation columns, reference is made to Chemical Engineer's Handbook, 5th Edition, edited by R.H. Perry and C.H. Chilton, McGraw-Hill Book Company, New York; Chapter 13, "Distillation" B.D. Smith et al., Page 13-3, The Continous Distillation Process.

As used herein, the term "evaporation column" refers to a column in which a liquid feed is directed to the column from its top, whereby more volatile components leave or evaporate from the liquid flowing downward with the rising steam stream.

As used herein, the terms "natural gas" and "natural gas" refer to a methane-containing fluid that is normally recovered from natural gas fields or crude oil sources.

As used herein, the terms "nitrogen-containing natural gas stream" and "nitrogen-containing hydrocarbon stream" mean a stream with a nitrogen content of 1 to 99 per cent.

77642 6

As used herein, the terms "natural gas liquids" and "higher hydrocarbons" refer to hydrocarbons having two or more carbon atoms.

As used herein, the term "ambient pressure" refers to the external pressure exerted on the heat pump circuit piping. Usually this ambient pressure is air pressure.

Brief description of the drawing

Figure 1 is a flow diagram of a preferred embodiment of the process of the present invention.

Detailed Description A detailed description of the present invention will be made with reference to the drawing. Referring to Figure 1, stream 12 is a gaseous stream derived from the first separation of a feed stream from a gas field or source. The original feed stream contained nitrogen, methane, and higher hydrocarbons, and in the first separation, the mixture of all higher hydrocarbons was separated as a liquid. The gaseous stream 12 is the second part of the first separation and contains nitrogen, methane and somewhat higher hydrocarbons. Typically, stream 12 has a nitrogen content of 3 to 90 percent, a methane content of 10 to 97 percent, higher hydrocarbons has a content of 1 to 15 percent, and stream 12 has a temperature of 140 to 230 ° K. .

The stream 12 is cooled in the heat exchanger 40 by recirculating currents of at least 10 Kelvin, preferably at least 20 ° K, most preferably at least 30 ° K, to create a partially condensed stream 60 which is passed to a phase separator 42. The vapor portion 61 from the phase separator 42 , which contains mainly nitrogen and methane, is further cooled in the heat exchanger 41 by means of return flows, it is expanded by passing it to the valve 7 77642

Iin 64 and is fed as a feed stream 65 to a concentration column-45 which operates in an absolute pressure range of 200 to 450 psi. The liquid portion 62 from the phase separation device 42, which contains mainly methane and higher hydrocarbons, is expanded by passing it through a valve 63 and is further directed to an evaporation column 43 from the top of the column. The liquid portion 62 is preferably expanded so that its pressure drops by at least 50 psi as it passes through the valve 63. That is, the evaporator column 43 preferably operates at a pressure that is at least 50 psi lower than the pressure of the gaseous stream 12 from the first separation.

In the evaporation column 43, the liquid falls at the bottom of the column against the steam rising from the re-boiled liquid. The liquid at the bottom of the column is re-boiled in the bottom boiler 44, which can obtain its heat from any conventional source, such as a heat exchanger 40. The falling liquid and the rising steam come into contact with each other countercurrently, separating the introduced liquid into steam. as stream 66, and as a liquid which is removed from the column as stream 11 containing mainly higher hydrocarbons. As shown in Figure 1, stream 66 may be heated in heat exchanger 40 to exit stream 17.

Thus, by using phase separation and further separation of the liquid stream from the phase separation in the evaporation column, more hydrocarbons can be recovered, as shown in Figure 1 as a separate stream from methane 11. If this phase separation and separation in the evaporation column is not used, . The recovery of these higher hydrocarbons separately from methane is advantageous because the economic value of the higher hydrocarbons is greater when used as chemicals or chemical feedstocks than as fuel, and the recovery of these higher hydrocarbons together with 8,77642 methane necessarily results in their use as fuel rather than economically viable applications.

Although secondary methane removal is usually used to improve the recovery of natural gas liquids, in some applications it may be more advantageous to use such a double methane removal system to reduce process power requirements. For example, process calculations have shown that several percent savings in process power requirements can be achieved by using a double methane removal system while maintaining the same natural gas liquefaction.

The 45 column nitrogen-containing natural gas feed stream is separated into a nitrogen-enriched vapor from the top and a methane-enriched bottom liquid. The bottom liquid is removed from column 45 as stream 79 and heated in heat exchanger 51 by indirect heat exchange to condense the heat pump flow medium to form a partially vaporized stream 80 which is passed to a phase separator 52. The vapor fraction 81 from the phase separator 52 The liquid portion 95 from the phase separator 52 is expanded by allowing this portion to pass through the valve 56, and the liquid stream 67 is heated in space exchanger 41 to space 68, further heated in heat exchanger 40, and exited as stream 20, which is a methane product. The vapor from the top is removed from column 45 as stream 70 and cooled in heat exchanger 49 by indirect heat exchange to vaporize the heat pump flow medium to form a partially condensed stream 71 which is passed to phase separator 53. The liquid portion 72 from phase separator 53 is returned to column 45.

The steam portion 73 from the phase separator 53 is heated in heat exchanger 48 to space 74, further heated in heat exchanger 47 to space 75, and further heated in heat exchanger 41 to space 76, after which it is heated 9 77642 in heat exchanger 40 from which it exits as a nitrogen stream 23. This stream can be released into the air. or, more preferably, it can be recovered and used later in an improved gas recovery process or an improved oil recovery process for injection into fields or sources.

Column 45 is operated by a heat pump circuit, which takes heat from the top of the column and returns it to the bottom of the column. It is recommended that the heat pump also removes some heat from the middle stages of the column. The heat pump circuit is a closed loop and its mass does not depend on column 45. In the heat pump circuit, a mixture of nitrogen and methane is used as the heat pump flow medium. This mixture consists of 0.5 to 60 mole percent nitrogen and 99.5 to 40 mole percent methane, preferably 1 to 30 mole percent nitrogen and 99 to 70 mole percent methane, and most preferably 5 to 20 mole percent methane. mole percent nitrogen and 95 to 80 mole percent methane.

The embodiment of Figure 1 uses a two-stage or two-pressure heat pump circuit, which is the preferred embodiment and will be described in detail below. The heat pump flow medium at pressure 89 and ambient temperature is cooled in a heat exchanger 46 against a heating, recyclable heat pump flow medium to a cooled, pressurized space 90. The compacted heat pump flow medium 91 is then cooled in a heat exchanger 47 against a heating, recyclable heat pump flow medium, and most preferably against a heating, recoverable nitrogen flow.

The subcooled heat pump flow medium exiting the heat exchanger 47 is then divided into two parts. The first portion 92 is further cooled in heat exchanger 48 against a heating, recyclable heat pump flow medium, and most preferably against a heating, recoverable nitrogen flow. The first portion of the still-cooled heat pump flow medium exiting the heat exchanger 48 is expanded by passing it through a valve 54 to a pressure of at least ambient pressure and a flow of 97, and is passed through a heat exchanger 49 to partially condense the vapor at the top of the column 45. The first portion of the heat pump flow medium exits the heat exchanger 49 as a stream 82, is heated to space 83 by passing it through heat exchanger 48, is further heated in heat exchanger 47 to space 84, is further heated in heat exchanger 46 to space 85, then passed through compressor 57, where it is compressed. to medium pressure. The second part of the liquid of the under-cooled heat pump is expanded by passing it through the valve 55 to a pressure in the middle stages which is higher than the pressure to which the first part was expanded, and this second part stream 96 is passed through a heat exchanger 50. , which is a stream of steam taken from the middle of the column 45, to at least partially condense. This at least partially condensed stream 78 is then returned to column 45 as a cooled additional stream. The second portion of the heat pump flow medium exits the heat exchanger 50 as a stream 86, is heated by passing it through the heat exchanger 47 to the space 87, is further heated in the heat exchanger 46 to the space 88, and is connected to the first portion at medium pressures. This combined stream is then passed through a compressor 58 where it is compressed and exited as a stream 89 to begin its cycle again. Thus, it has been found that the pressure in the circuit of the heat pump is always equal to or higher than the ambient pressure. Vacuum conditions are avoided in the circuit, and the circuit does not occur without absorption.

Many advantages are achieved with the use of the heat pump medium mixture according to the present invention. Using a certain amount of nitrogen as the heat pump flow medium lowers the boiling point of the heat pump flow medium, allowing the separation column 45 to operate at lower pressures. The operation of the separation column 45

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11 77642 at which pressures lowers the condensation temperature of the stream 70 from the top of the column. Similarly, in the heat exchanger 49, the boiling temperature of the heat pump flow medium must also be lowered. The use of a certain amount of nitrogen in the heat pump flow medium lowers the boiling point of the liquid, but still maintains a positive pressure in the heat pump circuit at this point, thus avoiding vacuum conditions in the heat pump circuit.

By operating the column 45 at a lower pressure, a more efficient separation of the feed stream into its nitrogen and methane components is achieved, as either the number of separation zones required or the amount of reflux liquid is reduced, or a more efficient separation is achieved by a combination of the two. In addition, the column 45 can operate at a lower pressure than in a conventional single-column nitrogen-methane separation process, without the significant disadvantages resulting from the vacuum conditions within the heat pump. In the cross-column nitrogen and methane separation process of the present invention, the column can operate at lower pressures than would be required in a conventional, single column while maintaining a positive pressure at all points in the heat pump circuit. The pressure drop depends on the amount of nitrogen in the heat pump flow medium, and the drop can. . be of the order of 150 psi. In those situations where the nitrogen stream is not to be reused in nitrogen injection processes, the nitrogen released at lower pressure does not waste the process pressure energy.

Another advantage of the heat pump loop loop using the medium mixture of the present invention relates to the temperature patterns present in the condenser boiler 51 and the return of the liquid condensers 50 and 49 to be cooled. In situations where the separation on column 45 does not require very pure products, the vapor 70 from the top condenses at a certain temperature range and not at a constant temperature and the bottom liquid 79 evaporates at a certain temperature range, and at a constant temperature of 12 77642

Temperature. Similarly, the steam from the middle stages of the column, which is always a mixture, condenses over a certain temperature range. Thus, if the heat pump flow medium is a mixture that condenses in a certain temperature range and evaporates in a certain temperature range and not at constant temperatures, the use of countercurrent heat transfer in the boiler 51 and return to the cooled liquid condensers 50 and 49 the pressure levels in the heat pump loop decrease. The direct consequence of this is a reduction in the energy costs associated with the heat pump circuit, i.e. less energy is needed to produce a certain amount of return-cooled liquid in the separation in the column. The degree of power reduction can be estimated by noting that in an application where the stream from the top of the nitrogen column can contain five moles of methane, a heat pump consisting of a mixture of nitrogen and methane containing about 5 mole percent nitrogen and 95 mole percent methane can reduce percent compared to the power requirement when using 100% methane as the heat pump flow medium.

Table I shows typical process conditions obtained by computer simulation of the process of this invention. The stream numbers correspond to the stream numbers shown in Figure 1 and the notation C2 + denotes hydrocarbons having two or more carbon atoms.

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Table I

13 77642

Current Flow Absolutely. Thermal Composition (mole%) No. (lb mole / pressure state ° K N2 CH4 c2 + b) (psi) 12 5NIGHT 400 186. t 24.1 72.5 3.4 60 5000 400 174.6 24.1 72.5 3.4 61 4600 400 174.6 25.6 72.7 1.3 62 400 400 174.6 5.7 70.4 23.9

66 275 135 153.7 6.2 91.0 0.B

17 275 135 181.7 8.2 91.0 0.8 11 125 135 183.3 - 25.0 75.0 20 3693 231 181.7 7.5 90.4 2.1 23 907 400 181.7 99.5 0.5 - 65 4600 400 156.2 25.6 72.7 1.7 70 3756 400 122.2 99.1 0.9 - 71 3756 400 122.0 99.1 0.9 - 72 2849 400 122.0 99.0 1.0 - 73 907 400 122.0 99.5 0.5 - 77 2650 400 152.6 52.5 47.5 - 78 2650 400 148.7 52.5 47.5 - 79 5176 400 162.3 11.8 86.7 1.5 80 5176 400 167.0 11.8 86.7 1.5 81 1484 400 167.0 22.4 77.4 0.2 67 3693 231 154.3 7.5 90.4 2.1 85 873 32 308.0 7.0 93.0 - 88 467 165 308.0 7.0 93.0 - 89 1340 368 311.0 7.0 93.0 - 90 1340 368 182.0 7.0 93.0 - 91 1340 368 164.0 7.0 93.0 - 96 467 165 142.0 7.0 93.0 to 86 467 165 150.0 7.0 93.0 to 97 873 32 115.0 7.0 93.0 to 82 873 32 121.0 7.0 93.0 - Using the process of this invention, a feed stream containing nitrogen, methane and higher hydrocarbons can be more efficiently separated into these components. Although the invention has been described in detail with reference to one particular preferred embodiment, it should be noted that numerous other embodiments are within the spirit and scope of the claims.

Claims (13)

14 77642 1. A process for removing nitrogen from natural gas, in which the nitrogen-containing hydrocarbon stream is first separated into a liquid containing mainly hydrocarbons having two or more carbon atoms and steam containing mainly nitrogen and methane, and the process further separates the vapor stream into one or more concentration columns , characterized in that the process recovers substantially all hydrocarbons containing two or more carbon atoms contained in the nitrogen-containing hydrocarbon stream, the process comprising: A) cooling the steam after the first separation to at least 10 degrees Kelvin to partially condense it; B) passing the condensed portion to an evaporation column, where it is separated into a liquid containing mainly hydrocarbons having two or more carbon atoms and a vapor containing mainly methane; C) recovering the hydrocarbon liquid and methane vapor of step B); and D) passing the non-condensed portion to the concentration column (s), where it is separated into nitrogen and methane. Process according to Claim 1, characterized in that the steam obtained after the first separation is cooled to at least 20 degrees Kelvin. Process according to Claim 1, characterized in that the steam obtained after the first separation is cooled to at least 30 degrees Kelvin. li 15 77642 Process according to Claim 1, characterized in that the evaporation column operates at a pressure of at least 50 ps (approx. 345 kPa) lower than the pressure of the steam obtained from the first separation. Process according to claim 1, characterized in that said cooling of step A) is carried out at least in part by indirect heat exchange of the steam obtained from the first separation with methane steam from the evaporation column. Process according to claim 1, characterized in that the pressure of the concentration column (s) is kept below the pressure normally used, but avoiding vacuum conditions within the heat pump by process steps comprising: a) conducting said non-condensed part ..450 psi (about 1.3 ... 3.1 MPa); b) separating this feed stream in said column into nitrogen-enriched peak vapor and methane-enriched bottom liquid; (c) cooling the peak steam by indirect heat exchange, wherein the heat pump flow medium comprising 0.5 to 60 mole percent nitrogen and 99.5 to 40 mole percent methane evaporates to partially condense said peak steam; d) returning the condensed portion of the peak vapor to the column as a reflux liquid and the process of removing the uncondensed portion of the vapor from the peak as nitrogen; e) heating the bottom liquid by indirect heat exchange, wherein said heat pump medium condenses to partially vaporize said bottom liquid; and 16 7 7642 f) returning the vaporized portion of the bottom liquid to the column as refluxed vapor, and recovering the non-vaporized portion as the product-forming natural gas; wherein said heat pump medium is at or above ambient pressure at all times * Process according to claim 6, characterized in that said heat pump medium consists of 1 to 30 mol% of nitrogen and 99 to 70 mol% of methane. Process according to claim 6, characterized in that said heat pump medium consists of 5 to 20 mol% of nitrogen and 95 to 80 mol% of methane. A process according to claim 6, characterized in that said heat pump flowing medium circulates in a closed loop, comprising: 1. cooling the heat pump flow medium with a liquid obtained by indirect heat exchange; 2. dividing the flow medium of the cooled heat pump into a first part and a second part; 3. expanding the first part to a pressure at least equal to ambient pressure and heating the expanded first part by indirect heat exchange with the steam from the peak; 4. expanding the second part to an average pressure greater than the pressure at which the first part is inflated and heating the expanded second part by indirect heat exchange with the steam stream taken from the middle stages of the column; and 5. compressing the expanded first part and the second part with a compressor to form a heat pump flowing medium to be cooled in step 1). I! 17 77642 Process according to Claim 9, characterized in that the steam stream obtained from the middle stages of step 4) is at least partially condensed and returned to the column as a cooled stream. Process according to Claim 9, characterized in that at least the second, heated, expanded part is further heated before being compressed by the compressor by indirect heat exchange with the flow medium of the heat pump before it is expanded. Process according to Claim 9, characterized in that the heat pump flow medium is further cooled before being expanded by indirect heat exchange with the non-condensed part of the steam from the peak before this part is removed from the process. A process according to claim 9, characterized in that the first part is compressed by a compressor to said intermediate pressure, combined with the second part and the combined stream is compressed by a compressor to form a heat pump flow medium to be cooled in step 1). 77642 18